The first electrocardiogram (ECG) was invented by a Dutch physiologist, Willem Einthoven, in 1903, who used a string galvanometer to measure the electrical activity of the heart. Generations of physicians around the world have since used ECGs, in various forms, to diagnose heart problems and other potential medical concerns. Although the basic principles underlying Dr. Einthoven's original work, including his naming of various waveform deflections (Einthoven's triangle), are still applicable today, ECG machines have evolved from his original three-lead ECG, to ECGs with unipolar leads connected to a central reference terminal starting in 1934, to augmented unipolar leads beginning in 1942, and finally to the 12-lead ECG standardized by the American Heart Association in 1954 and still in use today. Further advances in portability and computerized interpretation have been made, yet the electronic design of the ECG recording apparatuses has remained fundamentally the same for much of the past 40 years.
Essentially, an ECG measures the electrical signals emitted by the heart as generated by the propagation of the action potentials that trigger depolarization of heart fibers. Physiologically, transmembrane ionic currents are generated within the heart during cardiac activation and recovery sequences. Cardiac depolarization originates high in the right atrium in the sinoatrial (SA) node before spreading leftward towards the left atrium and inferiorly towards the atrioventricular (AV) node. After a delay occasioned by the AV node, the depolarization impulse transits the Bundle of His and moves into the right and left bundle branches and Purkinje fibers to activate the right and left ventricles.
During each cardiac cycle, the ionic currents create an electrical field in and around the heart that can be detected by ECG electrodes placed on the skin. Cardiac electrical activity is then visually represented in an ECG trace by PQRSTU-waveforms. The P-wave represents atrial electrical activity, and the QRSTU components represent ventricular electrical activity. Specifically, a P-wave represents atrial depolarization, which causes atrial contraction.
P-wave analysis based on ECG monitoring is critical to accurate cardiac rhythm diagnosis and focuses on localizing the sites of origin and pathways of arrhythmic conditions. P-wave analysis is also used in the diagnosis of other medical disorders, including imbalance of blood chemistry. Cardiac arrhythmias are defined by the morphology of P-waves and their relationship to QRS intervals. For instance, atrial fibrillation (AF), an abnormally rapid heart rhythm, can be confirmed by the presence of erratic atrial activity or the absence of distinct P-waves and an irregular ventricular rate. Atrial flutter can be diagnosed with characteristic “sawtooth” P-waves often occurring twice for each QRS wave. Some congenital supraventricular tachycardias, like AV node re-entry and atrioventricular reentrant tachycardia using a concealed bypass tract, are characterized by an inverted P-wave occurring shortly after the QRS wave. Similarly, sinoatrial block is characterized by a delay in the onset of P-waves, while junctional rhythm, an abnormal heart rhythm resulting from impulses coming from a locus of tissue in the area of the AV node, usually presents without P-waves or with inverted P-waves within or shortly before or after the QRS wave. Also, the amplitudes of P-waves are valuable for diagnosis. The presence of broad, notched P-waves can indicate left atrial enlargement or disease. Conversely, the presence of tall, peaked P-waves, especially in the initial half, can indicate right atrial enlargement. Finally, P-waves with increased amplitude can indicate hypokalemia, caused by low blood potassium, whereas P-waves with decreased amplitude can indicate hyperkalemia, caused by elevated blood potassium.
Cardiac rhythm disorders may present with lightheadedness, fainting, chest pain, hypoxia, syncope, palpitations, and congestive heart failure (CHF), yet rhythm disorders are often sporadic in occurrence and may not show up in-clinic during a conventional 12-second ECG. Some atrial rhythm disorders, like atrial fibrillation, are known to cause stroke, even when intermittent. Continuous ECG monitoring with P-wave-centric action potential acquisition over an extended period is more apt to capture sporadic cardiac events that can be specifically identified and diagnosed. However, recording sufficient ECG and related physiological data over an extended period remains a significant challenge, despite an over 40-year history of ambulatory ECG monitoring efforts combined with no appreciable improvement in P-wave acquisition techniques since Dr. Einthoven's original pioneering work over a 110 years ago.
Electrocardiographic monitoring over an extended period provides a physician with the kinds of data essential to identifying the underlying cause of sporadic cardiac conditions, especially rhythm disorders, and other physiological events of potential concern. A 30-day observation period is considered the “gold standard” of monitoring by some, yet a 14-day observation period is currently deemed more achievable by conventional ECG monitoring approaches. Realizing a 30-day observation period has proven unworkable with existing ECG monitoring systems, which are arduous to employ; cumbersome, uncomfortable and not user-friendly to the patient; and costly to manufacture and deploy. An intractable problem is the inability to have the monitoring electrodes adhere to the skin for periods of time exceeding 5-14 days, let alone 30 days. Still, if a patient's ECG could be recorded in an ambulatory setting over a prolonged time periods, particularly for more than 14 days, the chances of acquiring meaningful medical information and capturing an abnormal event while the patient is engaged in normal activities are greatly improved.
The location of the atria and their low amplitude, low frequency content electrical signals make P-waves difficult to sense, particularly through ambulatory ECG monitoring. The atria are located either immediately behind the mid sternum (upper anterior right atrium) or posteriorly within the chest (left atrium), and their physical distance from the skin surface, especially when standard ECG monitoring locations are used, adversely affects current strength and signal fidelity. Cardiac electrical potentials measured from the classical dermal locations have an amplitude of only one-percent of the amplitude of transmembrane electrical potentials. The distance between the heart and ECG electrodes reduces the magnitude of electrical potentials in proportion to the square of change in distance, which compounds the problem of sensing low amplitude P-waves. Moreover, the tissues and structures that lie between the activation regions within the heart and the body's surface further attenuate the cardiac electrical field due to changes in the electrical resistivity of adjacent tissues. Thus, surface electrical potentials, when even capable of being accurately detected, are smoothed over in aspect and bear only a general spatial relationship to actual underlying cardiac events, thereby complicating diagnosis. Conventional 12-lead ECGs attempt to compensate for weak P-wave signals by monitoring the heart from multiple perspectives and angles, while conventional ambulatory ECGs primarily focus on monitoring higher amplitude ventricular activity, i.e., the R-wave, that, comparatively, can be readily sensed. Both approaches are relatively unsatisfactory with respect to the P-wave and related need for the accurate acquisition of the P and R-wave medically actionable data of the myriad cardiac rhythm disorders that exist.
Additionally, maintaining continual contact between ECG electrodes and the skin after a day or two of ambulatory ECG monitoring has been a problem. Time, dirt, moisture, and other environmental contaminants, as well as perspiration, skin oil, and dead skin cells from the patient's body, can get between an ECG electrode's non-conductive adhesive and the skin's surface. These factors adversely affect electrode adhesion which in turn adversely affects the quality of cardiac signal recordings. Furthermore, the physical movements of the patient and their clothing impart various compressional, tensile, bending, and torsional forces on the contact point of an ECG electrode, especially over long recording times, and an inflexibly fastened ECG electrode will be prone to becoming dislodged or unattached. Moreover, subtle dislodgment may occur and be unbeknownst to the patient, making the ECG recordings worthless. Further, some patients may have skin that is susceptible to itching or irritation, and the wearing of ECG electrodes can aggravate such skin conditions. Thus, a patient may want or need to periodically remove or replace ECG electrodes during a long-term ECG monitoring period, whether to replace a dislodged electrode, reestablish better adhesion, alleviate itching or irritation, allow for cleansing of the skin, allow for showering and exercise, or for other purpose. Such replacement or slight alteration in electrode location actually facilitates the goal of recording the ECG signal for long periods of time.
Conventionally, multi-week or multi-month monitoring can be performed by implantable ECG monitors, such as the Reveal LINQ insertable cardiac monitor, manufactured by Medtronic, Inc., Minneapolis, Minn. This monitor can detect and record paroxysmal or asymptomatic arrhythmias for up to three years. However, like all forms of implantable medical device (IMD), use of this monitor requires invasive surgical implantation, which significantly increases costs; requires ongoing follow up by a physician throughout the period of implantation; requires specialized equipment to retrieve monitoring data; and carries complications attendant to all surgery, including risks of infection, injury or death. Finally, such devices do not necessarily avoid the problem of signal noise and recording high quality signals.
Holter monitors are widely used for ambulatory ECG monitoring. Typically, they are used for only 24-48 hours. A typical Holter monitor is a wearable and portable version of an ECG that includes cables for each electrode placed on the skin and a separate battery-powered ECG recorder. The leads are placed in the anterior thoracic region in a manner similar to what is done with an in-clinic standard ECG machine using electrode locations that are not specifically intended for optimal P-wave capture but more to identify events in the ventricles by capturing the R-wave. The duration of monitoring depends on the sensing and storage capabilities of the monitor. A “looping” Holter (or event) monitor can operate for a longer period of time by overwriting older ECG tracings, thence “recycling” storage in favor of extended operation, yet at the risk of losing event data. Although capable of extended ECG monitoring, Holter monitors are cumbersome, expensive and typically only available by medical prescription, which limits their usability. Further, the skill required to properly place the electrodes on the patient's chest precludes a patient from replacing or removing the sensing leads and usually involves moving the patient from the physician office to a specialized center within the hospital or clinic.
U.S. Pat. No. 8,460,189, to Libbus et al. (“Libbus”) discloses an adherent wearable cardiac monitor that includes at least two measurement electrodes and an accelerometer. The device includes a reusable electronics module and a disposable adherent patch that includes the electrodes. ECG monitoring can be conducted using multiple disposable patches adhered to different locations on the patient's body. The device includes a processor configured to control collection and transmission of data from ECG circuitry, including generating and processing of ECG signals and data acquired from two or more electrodes. The ECG circuitry can be coupled to the electrodes in many ways to define an ECG vector, and the orientation of the ECG vector can be determined in response to the polarity of the measurement electrodes and orientation of the electrode measurement axis. The accelerometer can be used to determine the orientation of the measurement electrodes in each of the locations. The ECG signals measured at different locations can be rotated based on the accelerometer data to modify amplitude and direction of the ECG features to approximate a standard ECG vector. The signals recorded at different locations can be combined by summing a scaled version of each signal. Libbus further discloses that inner ECG electrodes may be positioned near outer electrodes to increase the voltage of measured ECG signals. However, Libbus treats ECG signal acquisition as the measurement of a simple aggregate directional data signal without differentiating between the distinct kinds of cardiac electrical activities presented with an ECG waveform, particularly atrial (P-wave) activity.
The ZIO XT Patch and ZIO Event Card devices, manufactured by iRhythm Tech., Inc., San Francisco, Calif., are wearable monitoring devices that are typically worn on the upper left pectoral region to respectively provide continuous and looping ECG recording. The location is used to simulate surgically implanted monitors, but without specifically enhancing P-wave capture. Both of these devices are prescription-only and for single patient use. The ZIO XT Patch device is limited to a 14-day period, while the electrodes only of the ZIO Event Card device can be worn for up to 30 days. The ZIO XT Patch device combines both electronic recordation components and physical electrodes into a unitary assembly that adheres to the patient's skin. The ZIO XT Patch device uses adhesive sufficiently strong to support the weight of both the monitor and the electrodes over an extended period and to resist disadherence from the patient's body, albeit at the cost of disallowing removal or relocation during the monitoring period. The ZIO Event Card device is a form of downsized Holter monitor with a recorder component that must be removed temporarily during baths or other activities that could damage the non-waterproof electronics. Both devices represent compromises between length of wear and quality of ECG monitoring. Neither is designed for a female-friendly fit or for recording of the atrial (P-wave) signals.
Personal ambulatory monitoring, both with smartphones or via adjuncts to smartphones, such as with a wirelessly-connected monitor or activity tracker, of varying degrees of sophistication and interoperability, have become increasingly available. For instance, McManus et al., “A Novel Application for the Detection of an Irregular Pulse using an iPhone 4S in Patients with Atrial Fibrillation,” Vol. 10(3), pp. 315-319 (March 2013), the disclosure of which is incorporated by reference, discloses obtaining pulsatile time series recordings before and after cardioversion using the digital camera built into a smartphone. An algorithm implemented as an app executed by the smartphone analyzed recorded signals to accurately distinguish pulse recordings during atrial fibrillation from sinus rhythm, although such a smartphone-based approach provides non-continuous observation and would be impracticable for long term physiological monitoring. Further, the smartphone-implemented app does not provide continuous recordings, including the provision of pre-event and post-event context, both of which are critical for an accurate medical diagnosis that might trigger a meaningful and serious medical intervention. In addition, a physician would be loath to undertake a surgical or serious drug intervention without confirmatory evidence that the wearer in question was indeed the subject of the presumed rhythm abnormality. Validation of authenticity of the rhythm disorder for a specified patient takes on critical legal and medical importance.
The AliveCor heart monitor, manufactured by AliveCor, Inc., San Francisco, Calif., provides a non-continuous, patient-triggered event monitor, which is worn on the fingertip. Heart rate is sensed over a single lead (comparable to Lead I on a conventional ECG) and recorded by an app running on a smartphone, such as an iOS operating system-based smartphone, such as the iPhone, manufactured by Apple Inc., Cupertino, Calif., or an Android operating system-based smartphone, manufactured and offered by various companies, including Google Inc., Mountain View, Calif.; Samsung Electronics Co., Ltd., Suwon, S. Korea; Motorola Mobility LLC, a subsidiary of Google Inc., Libertyville, Ill.; and LG Electronics Inc., Seoul, S. Korea. The Android operating system is also licensed by Google Inc. The app can send the data recorded by an AliveCor heart monitor from the smartphone to healthcare providers, who ultimately decide whether to use the data for screening or diagnostic purposes. Furthermore, as explained supra with respect to the McManus reference, none of these devices provides the context of the arrhythmia, as well as the medico-legal confirmation that would otherwise allow for a genuine medical intervention.
Similarly, adherents to the so-called “Quantified Self” movement combine wearable sensors and wearable computing to self-track activities of their daily lives. The Fitbit Tracker, manufactured by Fitbit Inc., San Francisco, Calif.; the Jawbone UP, manufactured by Jawbone, San Francisco, Calif.; the Polar Loop, manufactured by Polar Electro, Kempele, Finland; and the Nike+FuelBand, manufactured by Nike Inc., Beaverton, Oreg., for instance, provide activity trackers worn on the wrist or body with integrated fitness tracking features, such as a heart rate monitor and pedometer to temporally track the number of steps taken each day with an estimation calories burned. The activity tracker can interface with a smartphone or computer to allow a wearer to monitor their progress towards a fitness goal. These activity trackers are accessories to smartphones, including iOS operating system-based smartphones, Android operating system-based smartphones, and Windows Phone operating-system based smartphones, such as manufactured by Microsoft Corporation, Redmond, Wash., to which recorded data must be offloaded for storage and viewing.
The features of activity trackers can also be increasingly found in so-called “smart” watches that combine many of the features of activity trackers with smartphones. Entire product lines are beginning to be offered to provide a range of fitness- and health-tracking solutions. As one example, Samsung Electronics Co., Ltd., offers a line of mobile products with fitness-oriented features, including the Galaxy S5 smartphone, which incorporates a biometric fingerprint reader and heart rate monitor; the Gear 2 smart watch, which also incorporates a heart rate monitor; and the Gear Fit wearable device, which incorporates a heart rate monitor, real time fitness coaching, and activity tracker. The Galaxy S5 smartphone's heart rate monitor is not meant for continuous tracking, while the both the Gear 2 smart watch and Gear Fit wearable device must be paired with a smartphone or computer to offload and view the recorded data. Such a pairing requires the devices to be close to each other and makes data offload challenging when a smartphone or a computer are not at hand.
With all manner of conventional “fitness-oriented” devices, whether smartphone, smart watch, or activity tracker, quantified physiology is typically tracked for only the personal use of the wearer. Monitoring can be either continuous or non-continuous. The wearer must take extra steps to route recorded data to a health care provider; thus, with rare exception, the data is not time-correlated to physician-supervised monitoring nor validated. Furthermore, the monitoring is strictly informational and medically-significant events, such as serious cardiac rhythm disorders, including tachyarrhythmias and bradyarrhythmias, while potentially detectable, are neither identified nor acted upon.
In today's medical and legal environment, a mobile device, such as a smartphone, provides information that seldom can be translated into data that triggers surgery or drug therapy by a physician. In the case of a smartphone detecting a fast heartbeat, for example, such a detection and the information on the smartphone would neither be identified as truly related to the patient in question or would be deemed sufficient for subjecting a patient to surgery or potentially toxic drug therapy. Thus, such data that is available today is not actionable in a medically therapeutic relevant way. To make such data actionable, one must have recorded data that allows a specific rhythm diagnosis, and a vague recording hinting that something may be wrong with the heart will not suffice. Further, the recorded data must not only identify the heart-related event of concern, but the signals before and after the event, which provides critical medical information for a physician to diagnose the disorder specifically. Finally, the recorded data must be made certifiable, so that the relationship of the recorded data to the patient that the physician is seeing is clear and appropriately identifiable as an event originating in the patient being examined. Establishing this relationship of data-to-patient has become a legally mandatory step in providing medical care, which heretofore has been impracticable insofar as one cannot merely rely upon a smartphone to provide legally sufficient identification of an abnormality with actionable data such that a patient undergoes a serious medical or surgical intervention commonly used in the management of heart rhythm disorders.
Further, conventional wearable sensors are generally poorly-suited for continuous long-term monitoring due to inadequate power management and because of poor skin contact. Such devices can start trying to record physiological data, using up battery power, regardless of whether they are currently being worn by a person whose physiological data the sensors are intended to gather. As a result of this battery power drain, the effective monitoring time for which these sensors can be used is reduced. The power drain can further be exacerbated by the wearable devices performing functions other than physiological monitoring, such as interfacing with other devices and transferring collected data to these devices. For multi-purpose devices especially, such as smartphones, these additional activities can use up the majority of the battery power, leaving insufficient power for continuous long-term monitoring.
Therefore, a need remains for a low cost extended wear continuously recording ECG monitor attuned to conserving power and capturing low amplitude cardiac action potential propagation for arrhythmia diagnosis, particularly atrial activation P-waves, and practicably capable of being worn for a long period of time, especially in patient's whose breast anatomy or size can interfere with signal quality in both women and men.
A further need remains for facilities to integrate wider-ranging physiological and “life tracking”-type data into long-term ECG and physiological data monitoring coupled with an onboard ability to cascade into the medical records and to the medical authorities appropriate medical interventions upon detection of a condition of potential medical concern.
A still further need remains for a low cost extended wear continuously recording ECG monitor attuned to capturing low amplitude cardiac action potential propagation for arrhythmia diagnosis, particularly atrial activation P-waves, practicably capable of being worn for a long period of time, especially in patient's whose breast anatomy or size can interfere with signal quality in both women and men, and that is able to interface with other devices that are distant from the monitor.